In many different situations, compressed or pressurized air is used to power various different types of equipment. For example, in mines, air-powered rock drills, tugger hoists, shaft muckers, chute gates and vent doors are commonly used. Air-powered equipment is also used in other locations, such as manufacturing facilities and repair garages. Air power is used because it is a convenient and efficient method of powering tools in locations where other forms of power might be more awkward, or simply unavailable.
Air-powered equipment usually includes motors or other configurations containing mechanical parts, pistons, bores and the like which move relative to one another. Unless lubricated, such moving components will suffer wear as a result of rubbing against one another. Thus, typically, the manufacturers of air-powered equipment include a lubricating spray in the air stream upstream of the equipment. This creates an oil mist or fog which is carried by the air stream into the equipment to lubricate the various moving components.
As the air stream moves through the equipment and is exhausted, it carries the oil mist out of the air-powered equipment, through the exhaust, and into the air space to which the air exhaust vents. Equipment operators and other persons located in this air space are adversely affected by the oil mist that is present in the air that they are breathing.
In addition, air powered equipment can be quite noisy. The volume of the noise increases as the size of the motor increases. For example, in mining applications, jumbo rock drills are used. These drills are often so noisy that exposure to the noise is very uncomfortable, and prolonged exposure to such noise can adversely affect the hearing of people exposed.
In the past, muffler devices have been designed for the purpose of reducing or silencing the noise being emitted by air powered equipment. However, such devices have typically been found to be inadequate. Specifically, the process of venting compressed air through the exhaust tends to cause water vapour in the compressed air to freeze and form ice crystals inside the sound muffling device. As a result, ice tends to accumulate on the inside of the muffling device fairly quickly, gradually blocking air flow, thus rendering both the muffling device and the air-powered equipment ineffective.
U.S. Pat. No. 4,079,809 issued Mar. 21, 1978 to Visnapuu discloses an air motor muffler comprising a rigid muffler body and flexible baffles therein, the baffles being composed of neoprene or similar material. The baffles are arranged in series, and alternate between having edge holes and center holes which allow air to pass through the baffle, thus muffling the noise from the motor. The baffles are flexible, so that when the compressed air is vented through the muffler, the baffles vibrate, dislodging any ice crystals from the baffles and preventing the accumulation of ice. The air being exhausted then travels through a narrowed nozzle to the atmosphere.
The device of Visnapuu suffers from a number of defects. First, though the baffles are resistant to ice accumulation, ice may still accumulate on the body of the Visnapuu device in sufficient amounts to reduce or completely block airflow through the device. Second, the outlet of the Visnapuu device is substantially smaller than the inlet. Such a configuration tends to amplify the sound of passing compressed air pulses, producing white noise. Thus, the Visnapuu device may actually create noise as air passes through the muffler. Finally, the Visnapuu device allows oil mist to be discharged into the air, which can negatively affect personnel present near the device.
U.S. Pat. No. 4,299,305 issued Nov. 10, 1981 to Eriksson teaches an exhaust air muffler for silencing sound from air or gas outlets. The muffler comprises an outer tube or flexible hose which contains an inner sound absorbing body. The sound absorbing body may be composed of various types of flexible materials, such as foam plastic surrounded by a stocking, a body of spun or pleated thread, a large number of thin longitudinal threads or a large number of inwardly directed bundles of fibers. In the Eriksson device, both the outer tube and inner sound absorbing body may be flexible, so as to be deform when impacted by the compressed air being exhausted so as to prevent the formation of ice in the muffler.
In addition, Eriksson teaches that oil mist will tend to condense on the inner surface of the outer tube, and on the surface of the inner sound absorbing body. According to Eriksson, the condensed droplets eventually form into larger drops and are transported by the airflow within the muffler toward the outlet end of the muffler, where they drip out of the muffler, without being expelled into the air as a mist and creating a breathing hazard.
However, the muffler of Eriksson may not effectively prevent oil mist from being exhausted into the surrounding air. Specifically, while oil droplets may condense on the inner surfaces of the muffler, the droplets will tend to be re-misted each time air under pressure is vented through the muffler. This remisted oil is then vented through the muffler into the air. Therefore, the Eriksson device is inadequate for preventing oil mist from being released into the air.